The present invention relates to a method for ensuring the run of the web in the multi-cylinder dryer of a papermachine. The invention relates also to a device for carrying out the above method.
The multi-cylinder dryers of a papermachine have employed a so-called twin-wire run at the downstream end of a drying section. In the twin-wire run, the wire is supported against the jackets of cylinders included in two tiers of cylinders by means of two wires, one running along a tortuous path against the jackets of cylinders of an upper tier and the other against the jackets of cylinders of a lower tier. When passing over from one tier of cylinders to the other, the web travels unsupported. At machine speeds of more than 800 m/min, the air currents produced by the web and moving parts of the machine cause fluttering of the web in these open spaces. The fluttering leads to web breakups at the upstream end of such cylinder arrays, as the strength characteristics of the web are still poor due to a high water content.
Efforts have been made to resolve this problem by using a single-wire run, wherein the open runs of a web are eliminated and the web travels supported all the time by one and the same dryer wire between cylinders included in two tiers. The drying effect of those single-tier cylinders, whereat the wire at this point lies between the web and the cylinder, is negligible as the wire prevents the transfer of heat from the cylinder to the web. Indeed, in the most recent machines, such cylinders have been replaced with suction or vacuum rolls, and this has resulted in improved machine operating characteristics and the threading ropes have become unnecessary.
Originally, the single-wire groups generally used to comprise just two or three upstream drive groups of a machine, but their number has been increased as the machine speeds have increased. Some recent machines lack completely the twin-wire cylinder groups. A drawback in a single-wire run is the increased length of a dryer section, leading to the increased length of a machine hall and, thus, to the increased factory building costs. On the other hand, a drawback affecting the paper quality is that, in a single-wire run, heat is always supplied to the web from the same side of paper, resulting in possible defects in paper (curling).
As a summary of the above alternatives, it can be said that the benefits of a twin-wire run include two-sided drying operation and a short dryer section, but the drawbacks include poorer running characteristics at high machine speeds. The advantages and disadvantages of a single-wire run are essentially opposite relative to the above.
In addition, for example U.S. Pat. No. 3,753,298 discloses a machine configuration, which employs a twin-wire run but in which the web is all the time supported by either one of the dryer wires. This is achieved by passing the dryer wires by way of guide rolls mounted between the dryer cylinders in such a manner that, during the passage between a dryer cylinder and a guide roll, the dryer wire always runs tangentially to the other guide roll and the other dryer wire wrapping there around, whereby the web can be transferred from one dryer wire to another at these points without open draws. A weakness of the solution disclosed in the cited publication is that the run of a web against a dryer wire between dryer cylinders and wire guide rolls is not secure. Thus, the pressure differences prevailing in pockets defined by dryer cylinders and wire sections, the air currents produced thereby and, on the other hand, the adhesion forces between the web and cylinder surfaces detach the web from the dryer wires. Thus, the open, unsupported web is again susceptible to wrinkling and, at sufficiently high running speeds, this again leads to web breakups.
Naturally, attempts have been made to improve the operating characteristics of such a machine configuration by providing multi-cylinder dryers with air current controlling and/or producing structure at suitable locations. This type of solutions have been disclosed for example in Finnish Patents 68279 (Patent Application 841167) and 76142 (Patent Application 854494). The passage of the web against the dryer wire is secured by using vacuum developing blow boxes. However, the blow box assemblies and nozzle designs proposed in the above references require very large overall air quantities for a desired effect. These air quantities are typically about 2000-2400 m.sup.3 /h per pocket. In terms of energy efficiency, this is undesired and leads to very large diameters in compensation air manifolds as well as highly complicated and expensive air circulation systems.